1. Field of the Invention
The field of the present invention generally relates to improvements in methods of magnetization preparation for collection of magnetic resonance imaging data and to improvements in methods of data collection of magnetic resonance imaging data from physiological tissue. In certain preferred embodiments, the present invention is directed to improved methods for performing magnetic resonance angiography.
2. Description of the Background
Magnetic resonance imaging (MRI) is a method of producing extremely detailed pictures of body tissues and organs that employs the inherent magnetic properties of the physiological tissue. MR angiography (MRA) is an MRI study of the vasculature for the purpose of detecting and diagnosing various disorders of the vasculature.
The scheme for magnetization of tissue for collection of MRA data is carefully chosen to improve the quality of the image obtained and to isolate data from particular types of tissues.
Real time or prospective navigator methods can be utilized to reduce motion artifacts in MRI without requiring the patient to hold a breath. Navigator echo is executed immediately before the image echo and is processed in real time to estimate motion information, which is then used immediately to guide image echo acquisition. The delay from the start of navigator echo to the beginning of image echo has to be sufficiently short to allow accurate estimation of motion in image echo and correspondingly effective suppression of motion artifacts in MR images. For example, this delay time should be less than 30 milliseconds for effective reduction of respiratory motion artifacts. An important field of application of the real time navigator method is cardiac imaging, including black blood imaging such as MRI of vessel wall and cardiac chamber, and white blood imaging such as cine MRI of the heart and coronary MRA.
Balanced steady-state free precession (SSFP) data acquisition provides short repetition time (TR) and high signal-to-noise ratio (SNR) for three dimensional (3D) coronary MRA and has been demonstrated to be advantageous over spoiled gradient echo approaches. Most previous SSFP 3D coronary MRA studies were performed within a breathhold. Recently, a navigator respiratory gating technique has been incorporated to overcome breathhold-related limitations. Because preparatory RF pulses (RFs) are required to drive spins into steady state prior to data acquisition, the reported navigator SSFP coronary sequences executed the navigator and fat saturation pulses before the preparatory RFs (Spuenturp, Bornert, Botnar, Groen, Manning, and Stuber, Invest. Radiol. 37:637-642, 2002).
In general, the separation between the navigator echo and the first image echo is approximately 100 ms. That delay may be too long to provide accurate motion information, leading to artifacts in the collected data. Furthermore, the separation between the fat saturation and the image echoes makes the fat suppression dependent on the field inhomogeneity, thereby degrading its effectiveness. Finally, the number of preparatory RFs is limited to approximately 20, which may not be a large enough number for adequate SSFP signal contrast and sufficient reduction of signal oscillation in the case of a set of linearly ramped preparatory RF pulses.
Additional applications of MRI techniques include the use of contrast agents to improve signal detection and, specifically, contrast-enhanced magnetic resonance angiography (CEMRA) has become the method of choice for performing MRA. A typical CEMRA examination completes in 20-30 seconds and requires timing data acquisition to contrast arrival using a test injection or real-time monitoring. These timing procedures complicate the task of performing CEMRA and, though quite reliable, are not without possible error. Time-resolved data acquisition methods which capture the passing of the contrast bolus through the arteries, organ, and veins of interest have the advantage of eliminating the need for bolus timing while simultaneously mapping blood flow dynamics. However, typical 3D k-space sampling of the required high spatial frequency for depicting vascular details takes too long (10-20 seconds) to provide acceptable temporal resolution.
By discarding the sampling of depth resolution, 2D projection MRA can be completed in one second, thereby providing adequate temporal resolution with high in-plane resolution in a manner similar to conventional projection X-ray digital subtraction angiography (DSA). Such 2D projection magnetic resonance digital subtraction angiography (MRDSA) has been utilized for visualizing arterial venous malformation, resolving arteries in distal extremities, and providing timing information for bolus chase data acquisition in peripheral MRA. However, the lack of depth resolution limits the clinical utility of the 2D projection MRDSA.
Since the signal on 3D MRA is largely located in the center of k-space and since sparse bright voxels in MR angiograms allow undersampling in k-space, time resolved imaging of contrast kinetics (TRICKS) methods have been developed to provide adequate temporal resolution for 3D MRA while preserving high spatial resolution. Particularly, the radial k-space sampling (PR TRICKS) allows natural azimuthal undersampling and sliding window reconstruction. The PR TRICKS approach provides adequate temporal resolution with high 3D spatial resolution, promising vast clinical potential.
It should be noted that the sliding window expands to about 20 seconds for sampling a full, high-resolution k-space data set. Furthermore, the radial sampling is limited to sampling one straight spoke per RF excitation, which results in suboptimal SNR efficiency. Spiral sampling can substantially shorten the scan time and also allows azimuthal undersampling and sliding window reconstruction. Thus, there has been a recognized need in the medical imaging community for a data collection scheme that allows for improved temporal and spatial resolution.
Generally, SSFP imaging provides high SNR and fast speed and is being adopted in many clinical applications with additional magnetization preparation to enhance desired image contrast. When magnetization preparation pulses are inserted into an SSFP sequence, data acquisition becomes segmented and view ordering is required to maximize the signal contrast during the acquisition of the center of k-space. However, the repetition-to-repetition changes in phase encoding gradients cause spin phase variations associated with gradient-induced eddy currents and consequently lead to artifacts in SSFP imaging. The changes in phase encoding gradients from one repetition to the next have to be kept small to minimize these artifacts.
Bright background signal is also a major concern in SSFP CEMRA because of the T2/T1 tissue contrast. Such a bright background signal can be suppressed by adding a magnetization preparation into the SSFP sequence. A periodic insertion in the RF train of an inversion pulse, immediately followed by preparatory RFs with disabled data acquisition (disdacqs) will suppress tissues with long T1 relaxation. However, the temporal variations in blood and background signals after each inversion pulse may cause image artifacts and abate background suppression. These problems may be alleviated by matching the order of sampled k-space views to the variation in signal. The MRA field generally recognizes that an improved view order should satisfy two competing demands: 1) the central portion of k-space is acquired during the background signal nulling to optimize background suppression; and 2) the sampling trajectory in the phase and slice encoding space (kykz-space) is smooth to minimize artifacts associated with gradient induced eddy currents.
Further clinical MRA applications employ multiple RF channel parallel imaging using coil sensitivity encoding (SENSE). The standard procedure for performing SENSE consists of: 1) acquisition of a fully sampled reference scan for coil sensitivity estimation; and 2) accelerated acquisition of an undersampled scan for the targeted application. The reference scan for coil sensitivity increases the total scan time and may cause misregistration artifacts if the patient moves between the reference scan and the accelerated data acquisition. It is highly desirable to eliminate the reference scan by estimating the coil sensitivities from the accelerated acquisition itself.
Such self-calibration techniques have been developed for a parallel imaging variant—SMASH. In the Cartesian sampling trajectory case, additional views in the central region of k-space are acquired to construct a low resolution image with minimal wrapping artifacts. In the spiral imaging trajectory case, the high sampling density near k-space center naturally allows the reconstruction of a low resolution image with little artifacts. No additional spiral interleaf is required. Thus, there has been a clear need in the prior art for an improved parallel imaging variant SENSE technique that provides for rapid image collection and appropriate calibration without additional scans.
Time-resolved high resolution 3D MRI generates a huge amount of data for reconstruction. Multiple RF coil reception further extends the amount of computation for reconstruction. In the case of data sampled on non-Cartesian trajectories, such as spiral, reconstruction computation is substantially increased by regridding and off-resonance correction.